Fix bug in Cellwise FocalStrategy traversal.

The traversal should begin at the minimum row of the analysis area,
not at row 0 of the input raster.  This can cause fatal errors when
the analysis area extent has a y > 0.
Add benchmarks for focal operations on tiled rasters.

This checkin adds benchmarks for some focal operations on tiled rasters,
as well as an additional benchmark using a larger neighborhood.
Merge pull request #296 from joshmarcus/topic/focal_bug

Resolve bug with focal ops on tiled rasters and add benchmarks.
Add comments to color utilities.
Add documentation to RenderPNG and color classes.
Add default color ramps and a simple raster rendering operation.
Merge pull request #297 from joshmarcus/topic/color_rendering

Improve usability of raster rendering operations.
Fixing depenency for demo project.
Merge pull request #298 from lossyrob/pullreq/demo-fix

Fixing depenency for demo project.
Improve SimpleRenderPng operation and add tutorial example.
Merge pull request #299 from joshmarcus/improve_simple_png

Improve SimpleRenderPng operation and add tutorial example.